Transcript Anomalous X-ray Pulsars and Soft Gamma-ray Repeaters Sandro Mereghetti INAF - IASF Milano Topics in X-ray astronomy - Tuebingen February 2004 S.Mereghetti.

Anomalous X-ray Pulsars
and
Soft Gamma-ray Repeaters
Sandro Mereghetti
INAF - IASF Milano
Topics in X-ray astronomy - Tuebingen February 2004
S.Mereghetti
OUTLINE
Short review of AXP and SGR properties
AXP / SGR relationship
2 recent results
XMM-Newton > Variability in AXP 1E1048
INTEGRAL > Spectral evolution in bursts from SGR 1806
Topics in X-ray astronomy - Tuebingen February 2004
S.Mereghetti
NORMAL x-ray pulsars are
rotating magnetized neutron stars
1) In binary systems powered by accretion from a companio
star
e.g. Vela X-1, Cen X-3
Periods from 60 ms to a few 1000 s
2) rotation powered radio pulsars
e.g. Crab , Geminga, PSR 1957+20
Periods from 1.5 ms - a few seconds
Topics in X-ray astronomy - Tuebingen February 2004
S.Mereghetti
AXP in the context:
Accreting pulsars
Maximum X-ray Luminosity (erg/s)
Most accreting
pulsars are in
massive binaries
AXP are only
found in the
narrow period
range 5-12 s
Spin Period (s)
Topics in X-ray astronomy - Tuebingen February 2004
S.Mereghetti
Topics in X-ray astronomy - Tuebingen February 2004
S.Mereghetti
AXP PROPERTIES
No evidence for massive companion stars
limits on ax sin i from timing
limits on Fx/Fopt from optical/IR observations
Period of a few seconds
(6-12 s)
Almost steady spin down
Very soft X-ray spectrum
kTBB < 0.5 keV
 ph > 3-4
Topics in X-ray astronomy - Tuebingen February 2004
S.Mereghetti
AXP have very soft X-ray spectra
AXP
Topics in X-ray astronomy - Tuebingen February 2004
S.Mereghetti
AXP PROPERTIES
2 ( or 3 ? ) are in SNRs
34
36
-1
X-ray luminosity Lx = 10 - 10 erg s
Lx > rotational energy loss for a neutron star
Topics in X-ray astronomy - Tuebingen February 2004
S.Mereghetti
(Possenti et al. 2002)
X-RAY LUMINOSITY
ROTATION-POWERED PULSARS
SPIN - DOWN ENERGY LOSS
Topics in X-ray astronomy - Tuebingen February 2004
S.Mereghetti
Operational definition of AXP
a spinning down pulsar,
with a soft X-ray spectrum,
apparently not powered by accretion from a (massive)
companion star,
and with luminosity larger than the rotational energy loss
(assuming a neutron star)
Topics in X-ray astronomy - Tuebingen February 2004
S.Mereghetti
AXP census
P
(s)
4U 0142+61
1E 2259+5867
1E 1048-5937
1E 1841-045
AX J1845-03
RXS 1708-40
CXO J0110-72
XTE J1810-197
dP/dt
(10-11 s/s)
8.7
0.05
6.4
11.8
7
0.2
2-3
4
11
8
5.5
Topics in X-ray astronomy - Tuebingen February 2004
2
2
1.8
CTB 109
Kes 73
G296+0.1, Var.
in SMC
Var.
S.Mereghetti
MODELS
ROTATIONAL ENERGY
ACCRETION
- isolated NS
- isolated WD
- very low mass companion
- ISM / molecular clouds
- disk around isolated NS
MAGNETIC ENERGY
?
?
- field decay
- enhanced thermal emission
Topics in X-ray astronomy - Tuebingen February 2004
S.Mereghetti
Isolated NS + accretion disk
Thorne-Zytkow object
(Van Paradijs et al. 1995, Ghosh et al. 1997)
fall back after SN explosion
(Chatterjiee et al. 2000, Alpar 2001)
capture of SNR ejecta by fast moving NS
(Marsden et al. 2000, 2001)
Topics in X-ray astronomy - Tuebingen February 2004
S.Mereghetti
Magnetar model
(Thompson and Duncan)
Isolated NS with B > 1014 Gauss
Emission powered by magnetic field decay and/or
enhanced cooling
Originally developed for SGRs where evidence for
high B is stronger due to large flares and bursts
Extended to AXP due to similar P and dP/dt values
Topics in X-ray astronomy - Tuebingen February 2004
S.Mereghetti
Soft Gamma-ray Repeaters
• Initially discovered as a peculiar class of GammaRay Bursts
• soft
• repeating
• About 5 currently known
(1 in the LMC)
• Not always active (long quiescent periods)
Topics in X-ray astronomy - Tuebingen February 2004
S.Mereghetti
SGRs vs. GRBs
Durations
150
10
836 GAMMA-RAY
BURSTS
10
16
12
75
8
50
4
25
2
FLUX, photons/cm s keV
42 SOFT GAMMA
REPEATERS
10
NUMBER OF EVENTS
NUMBER OF EVENTS
10
20
125
100
Spectra
10
10
10
10
10
10
10
10
10
0
0.00
0
0.01
0.10
1.00
10.00 100.00 1000.00
DURATION, SECONDS
Courtesy K. Hurley
Topics in X-ray astronomy - Tuebingen February 2004
10
10
1
0
-1
GAMMA-RAY BURST
-2
-3
-4
-5
-6
-7
-8
SOFT GAMMA REPEATER
-9
-10
-11
-12
-13
10
0
10
1
10
2
3
4
10
10
10
ENERGY, keV
5
10
S.Mereghetti
6
10
7
Energetics of SGRs
Short Bursts:
Peak Luminosity
Total Energy
~1038-1042 erg s-1
~1039-1042 erg
Giant Flares:
Peak Luminosity
Total Energy
~4 x 1044 erg s-1
~0.7-2 x 1044 erg
Topics in X-ray astronomy - Tuebingen February 2004
S.Mereghetti
Giant Flares
1979 March 5 from
SGR 0526-66
1998 August 27 from
SGR 1900+14
Feroci et al. 1999
Mazets et al. 1979, Cline et al. 1980
Topics in X-ray astronomy - Tuebingen February 2004
S.Mereghetti
Persistent X-ray emission from SGRs
• Lx
= 1035 -1036 erg /s
• Pulsations with periods
(1-10 keV)
5 - 10 s
• secular spin-down at 10-11 s/s
• power law (+ blackbody) spectra
VERY SIMILAR TO AXPs !!
Topics in X-ray astronomy - Tuebingen February 2004
S.Mereghetti
AXP
Topics in X-ray astronomy - Tuebingen February 2004
S.Mereghetti
(Kaspi et al. 2003)
SGR-like
activity in the
AXP
1E2259+586
Topics in X-ray astronomy - Tuebingen February 2004
S.Mereghetti
bursts have
Lpeak: 1036-4 1038 erg/s
Change in pulse
morphology
Glitch  = 4 10-6
(Kaspi et al. 2003)
Topics in X-ray astronomy - Tuebingen February 2004
S.Mereghetti
AXP=SGR ?
Only observational selection effects introduced a
distinction between these sources belonging to the same
class of objects:
in AXP the quiescent, pulsating emission was discovered first
SGR were discovered through their bursts
Topics in X-ray astronomy - Tuebingen February 2004
S.Mereghetti
OUTLINE
Short review of AXP and SGR properties
AXP / SGR relationship
2 recent results
XMM-Newton > Variability in AXP 1E1048
INTEGRAL > Spectral evolution in bursts from SGR 1806
Topics in X-ray astronomy - Tuebingen February 2004
S.Mereghetti
First evidence for significant variability in 1E 1048-59
(Mereghetti et al. 2004)
2 EPIC
XMM-Newton
observation
Topics in X-ray astronomy - Tuebingen February 2004
S.Mereghetti
The pulsed fraction decreased while the flux increased
2000
2003
89 %
53 %
Spectrum
did not
vary
BB+PL
kT=0.6 keV
=3
Topics in X-ray astronomy - Tuebingen February 2004
S.Mereghetti
4U 0142+61
SAX
(Israel et al 1999)
Power law
photon index = 3.9
+
Blackbody
kT = 0.4 keV
Topics in X-ray astronomy - Tuebingen February 2004
S.Mereghetti
Most AXP require 2 component model PL + BB
Phot.index
kTBB
RBB
NH 1022
1E 1048-59
2.9
0.63 keV
0.4 d3 km
1.0
4U0142+61
3.9
0.40 keV
1.8 d1 km
1.1
1E 2259+58
3.6
0.41 keV
2.6 d4 km
0.9
RXS1708-40
2.6
0.46 keV
7.9 d8 km
1.4
AXJ1845-00
-
0.64 keV
3.9 d15 km
6
1E 1841-0045
3.0
-
Topics in X-ray astronomy - Tuebingen February 2004
-
2
S.Mereghetti
Are the two spectral
components related
to distinct emitting
regions and/or
physical processes ?
Topics in X-ray astronomy - Tuebingen February 2004
S.Mereghetti
(Oezel, Psaltis, Kaspi 2001)
small energy
dependence of
pulsed fraction
requires ad hoc
tuning of the BB and
PL components
Topics in X-ray astronomy - Tuebingen February 2004
S.Mereghetti
Despite the large flux variation the
spectral shape did not vary
BB+PL in both observations
kT = 0.6 keV
phot. Index = 3
... these are the typical parameters seen in this source
Topics in X-ray astronomy - Tuebingen February 2004
S.Mereghetti
The pulsed fraction decreased while the flux increased
2000
2003
89 %
53 %
BB and PL are not physically distinct components
Topics in X-ray astronomy - Tuebingen February 2004
S.Mereghetti
OUTLINE
Short review of AXP and SGR properties
AXP / SGR relationship
2 recent results
XMM-Newton > Variability in AXP 1E1048
INTEGRAL > Spectral evolution in bursts from SGR 1806
Topics in X-ray astronomy - Tuebingen February 2004
S.Mereghetti
SGR1806-20 entered a new period of activity in July 2003
An INTEGRAL ToO observation started on 3 September
2003, while the source was still active
INTEGRAL continued to observe SGR 1806-20 (l = 9.99
deg, b = -0.24 deg) during the Galactic Center Deep
Exposre (GCDE) until mid October
24 bursts were detected by IBIS in real time by the
INTEGRAL Burst Alert System (IBAS) and confirmed later
by off-line analysis
Topics in X-ray astronomy - Tuebingen February 2004
S.Mereghetti
24 bursts from SGR 1806-20 have been
detected with the INTEGRAL Burst Alert
System.
Number of bursts/day
GCDE
ToO
INTEGRAL Julian Day
Topics in X-ray astronomy - Tuebingen February 2004
S.Mereghetti
GCDE Bursts
Topics in X-ray astronomy - Tuebingen February 2004
S.Mereghetti
3-20 keV
15-40 keV
40-100 keV
JEM-X
Yoff = -0.97º
Zoff = -2.22º
IBIS/ISGRI
Fluence (15-100 keV)
2.5×10-8 erg cm-2
100-200 keV
Topics in X-ray astronomy - Tuebingen February 2004
S.Mereghetti
Spectral Analysis
15-100 keV IBIS/ISGRI spectra
of the bursts with more than 500
net counts
Optically
Thin
Thermal
Bremsstrahlung model provides
good fits (power-law, blackbody,
Band GRB model are ruled out)
kT ÷ 32-42 keV
Conversion factor (15-100 keV,
<kT> = 38 keV) 1 count s-1 =
1.5x10-10 erg cm-2 s-1
Topics in X-ray astronomy - Tuebingen February 2004
S.Mereghetti
INTEGRAL Log N- Log P
(Peak Flux distribution)
INTEGRAL Log N- Log S
(Fluence distribution)
Topics in X-ray astronomy - Tuebingen February 2004
S.Mereghetti
THE SGR BURSTS OBSERVED BY IBIS ARE
NORMAL IN MOST RESPECTS
Durations, energy spectra are typical
However, the fluences are very low,~1.5x108 erg/cm2 , 25-100 keV
These are the among the weakest bursts seen
from this SGR; thanks to imaging, we are
certain that the source is indeed SGR1806-20
Topics in X-ray astronomy - Tuebingen February 2004
S.Mereghetti
IBIS (20-40 keV)
6 sigma detection
2-3 mCrab source
Topics in X-ray astronomy - Tuebingen February 2004
S.Mereghetti
(INTEGRAL CP data ~ 1 Msec, courtesy Ada Paizis)
The MAGNETAR model predictions
Highly magnetized (B~1015 G),
slowly rotating (P~ 5-8 s)
neutron stars
e+e- plasma
Bursts are triggered by a
sudden
shift
in
the
magnetospheric
footpoints
driven by a fracture in the
neutron star crust
The radiation originates from
the cooling of an optically thick
pair-photon plasma
Topics in X-ray astronomy - Tuebingen February 2004
Thompson & Duncan (1995)
S.Mereghetti
For typical (~0.1 s long) bursts:
No signifcant spectral evolution predicted and in general NOT observed
up to now (e.g. Fenimore et al. 1994, Kouveliotou et al. 1987)
•
Soft-to-hard evolution
Topics in X-ray astronomy - Tuebingen February 2004
Photon Index
SGR 1900+14: an exception
Two
peculiar
bursts
of
intermediate duration (~1 s)
and and with hard (kT~100
keV) spectra
Time (seconds since trigger)
S.Mereghetti
Woods et al. (1999)
Spectral Evolution of weak bursts with INTEGRAL
15-40 keV
40-100 keV
Götz et al., 2004, A&A submitted
Topics in X-ray astronomy - Tuebingen February 2004
S.Mereghetti
Hardness-Intensity Anticorrelation with INTEGRAL
Hardness Ratio
(bursts with more than 200 net counts)
Counts/s
Topics in X-ray astronomy - Tuebingen February 2004
Götz et al., 2004, A&A submitted
S.Mereghetti
Conclusions
1) XMM / EPIC detected the first significant
variation in the flux and pulsed fraction of the
AXP 1048
the spectral invariance is a further evidence that the PL+BB
spectral decomposition does not have a physical meaning
2) INTEGRAL / IBIS detected the first evidence
for spectral evolution of fain SGR bursts as well
as a hardness intensity anticorrelation
these properties are not (yet) foreseen in the magnetar
model
S.Mereghetti
Topics in X-ray astronomy - Tuebingen February 2004